Wireless communication systems have been widely deployed to provide various types of communication services such as voice or data. In general, a wireless communication system is a multiple access system that supports communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power, etc.). The multiple access system can be categorized into orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) depending on whether orthogonality is guaranteed.
As examples of the OMA, frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), spatial domain multiple access (SDMA) are included. For each of them, the multiple access is performed in a frequency domain, time domain, code domain, and spatial domain.
Meanwhile, as a multiple access scheme in a power domain, the NOMA can be interconnected with the OMA, whereby it is possible to improve spectral efficiency.